The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the separation of fat and water signals in MR images produced using the Dixon method.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. An NMR signal is emitted by the excited spins, and after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
Materials other than water, principally fat, are found in biological tissue and have different gyromagnetic ratios. The Larmor frequency of protons in fat is shifted approximately 225 Hz from those of protons in water in a 1.5 Tesla polarizing magnetic field B0. The difference between the Larmor frequencies of such different species of the same nucleus, viz., protons, is termed chemical shift, reflecting the differing chemical environments of the two species.
Often it is desired to xe2x80x9cdecomposexe2x80x9d the NMR image into its several chemical shift components. In the exemplary case of protons, which will be used hereafter for illustration, it may be desired to portray as separate images the water and fat components of the subject. One method of accomplishing this is to acquire two images S0 and Sxe2x88x921 with the fat and water components of the images in phase, and out of phase by xcfx80 radians, respectively (the xe2x80x9cDixonxe2x80x9d technique). Adding and subtracting these images provides separate fat and water images. The phase shift between the fat and water components of the images may be controlled by timing the RF pulses of the NMR sequence so that the signal from the fat image evolves in phase with respect to the water by the proper angle of exactly xcfx80, before the NMR signal is acquired.
In the ideal case above, the frequency of the RF transmitter is adjusted to match the Larmor frequency of the water. If the polarizing magnetic field B0 is uniform, this resonance condition is achieved through out the entire subject. Similarly, the out-of-phase condition (xcfx80 radians) for the fat component is achieved for all locations in the subject under homogeneous field conditions. In this case, the decomposition into the separate images is ideal in that fat is completely suppressed in the water image, and vice versa.
When the polarizing field is inhomogeneous, however, there are locations in the subject for which the water is not on resonance. In this case, the accuracy of the decomposition breaks down and the water and fat images contain admixtures of the two species. Field inhomogeneities may result from improper adjustment or shimming of the polarizing magnetic field B0, but are more typically the result of xe2x80x9cdemagnetizationxe2x80x9d effects caused by the variations in magnetic susceptibility of the imaged tissue, which locally distort the polarizing magnetic field B0. These demagnetization effects may be of short spatial extent but of conventional linear or higher order shimming techniques.
The influence of demagnetization may be accommodated, however, by a three-point Dixon imaging technique that uses three acquired images S0, S1 and Sxe2x88x921, with the phase evolution times adjusted so that the fat and water components of the images are in phase, out of phase by xcfx80, and out of phase by xe2x88x92xcfx80 respectively. The complex pixels in each of the three images after conventional reconstruction may be processed as described, for example, in U.S. Pat. No. 5,144,235 to produce a separate water and a separate fat image.
An important assumption in Dixon imaging is that the spectral composition of living tissues is made of two distinct xcex4-peaks, one corresponding to the water proton resonance and the other corresponding to a loosely termed xe2x80x9cfatxe2x80x9d resonance peak. The latter is approximately 3.35 ppm, or 225 Hz at 1.5 Tesla field strength, apart from the water resonance frequency. In reality, the xe2x80x9cfatxe2x80x9d is composed of multiple spectral components. Table 1, lists the major spectral components of corn oil that was measured at 1.5 Tesla.
As illustrated in Table 1, the loosely-termed xe2x80x9cfatxe2x80x9d peak is actually composed of a series of peaks (Peaks 1-6) dominated by Peak #2 that corresponds to the methylene protons. In addition, there is actually another group of peaks (Peaks 7-9) whose frequencies fall more closely to the water resonance frequency. The signals from these protons that generate these latter peaks intermix with the xe2x80x9cwaterxe2x80x9d signals and are not separated properly by the Dixon method.
The present invention is an improved method for producing separate water and fat images. More specifically, the invention includes acquiring MRI data with an MRI system using a pulse sequence in which separate water and fat images may be reconstructed, producing a pixel shifted fat image from a reconstructed fat image which indicates fat signal components intermixed with a reconstructed water image, multiplying the pixel shifted fat image by a factor a, and subtracting the result from the water image. Fat signal components that are not separated from the water signal are emulated by the pixel shifted fat image and subtracted from the water image to remove them therefrom.